1. Field of the Invention
The present invention relates to an apparatus and methods for measuring flow velocity and other parameters in an earth formation using nuclear magnetic resonance techniques.
2. Discussion of Related Art
Well logging provides information about many important parameters that may be used to determine the “quality or characteristics” of an earth formation from wellbore measurements including, for example, the amount and producibility of hydrocarbons present in the formation. In addition to wireline logging, downhole formation sampling tools such s the Schlumberger Modular Formation Dynamics Tester (MDT) may be used to withdraw samples of fluids from earth formations for subsequent analyses. These analyses provide information to characterize physical properties of the formation fluids such as water and oil volume fractions, oil viscosity and water salinity. Knowledge of these and other physical characteristics may be needed to interpret wireline logs and to plan for the efficient exploitation of the reservoir.
Nuclear magnetic resonance (NMR) has become an invaluable tool for the characterization of materials and is widely used in geophysical exploration. NMR measurements, in general, are accomplished by causing the magnetic moments of nuclei (“spins”) in a formation to precess about an axis. The axis about which the nuclei precess may be established by applying a strong, polarizing, static magnetic field (B0) to the formation to align the proton spins. Next, a series of radio frequency (RF) pulses are produced so that an oscillating magnetic field B1 is applied. One common sequence of RF pulses that may be used is the error-correcting CPMG (Carr-Purcell-Meiboom-Gill) NMR pulse sequence. The frequency of the RF pulses may be chosen to excite specific nuclear spins, for example, water protons or Carbon-13 nuclei, in a particular region of interest of a sample.
Various NMR techniques have been used to measure formation parameters such as the spin-lattice relaxation time (T1), the spin-spin relaxation time (T2) and the diffusion coefficient (D). One important parameter is the permeability of the formation as this parameter may provide an indication of the difficulty that may be encountered in extracting the hydrocarbons from the formation. In some cases, NMR measurements can be used to create T2 distributions which represent pore size distributions in the formation, and permeability can be derived from these T2 distributions. However, this method of determining permeability suffers from several drawbacks and is not always applicable.
A more direct way to measure permeability uses measurements of induced flow rates of fluid in the formation. Several NMR-based flow measurement techniques have been proposed. For example, U.S. Pat. No. 6,518,758 to Speier et al. describes an NMR/MRI (magnetic resonance imaging) technique for measuring flow velocity in a wellbore based on frequency displacement (echo shape) of a received signal. U.S. Pat. No. 6,856,132 to Appel et al. describes another flow measurement technique that is based on creating at least two MRI images, at two different times, of the fluid flowing within the formation. The images are used to determine displacement of the fluid. In the methods of both the '758 and '132 patents, the spins are measured while they remain within the formation.
An apparatus incorporating remote detection NMR/MRI spectroscopy is disclosed in U.S. Pat. No. 7,061,237 to Pines et al. The '237 patent discloses an NMR apparatus including an encoding coil with a sample chamber, a supply of signal carriers, preferably hyperpolarized xenon, and a detector. According to the '237 patent, the apparatus allows the spatial and temporal separation of the encoding and detection steps, which allows the conditions of each step to be optimized depending on the subject of investigation. However, the '237 does not disclose the use of NMR in downhole applications, but instead discloses only surface applications, primarily in laboratories or medical facilities.